When I set up my alternator with a double pulley and twin belts last spring, I think it put my alternator off by about 150 rpm: I seem to get the same cruising speed of 7.5 kts at 1650 rpm that I achieved at 1800 the previous summer. I presume I can check with a photo tach, but I'm having trouble finding one I can borrow. Is there an adjustment screw on the gauge in the Pilothouse?

I noted someone in an earlier thread had installed a Tinytach. Is this another option for checking engine rpm?

Jim, when I changed my alternator, my tach reading changed 200 RPM even though I used the same pulley. It must have been due to the stator count on the alternator. The stator setting on the back of the tach was correct. I found an inexpensive photo tach for under $30 like this.

I used it to verify the tach's accuracy and found it to be within 40 RPM at cruise power of 1800. So it turns out my old alternator was giving me an inaccurate reading of 2000 RPM at cruise and now it's accurate. Knowing this made it worth the $30.

I'd send you mine to borrow but it'll cost more than $30 to send it there and back.

Jim, when I changed my alternator, my tach reading changed 200 RPM even though I used the same pulley. It must have been due to the stator count on the alternator. The stator setting on the back of the tach was correct. I found an inexpensive photo tach for under $30 like this.

I used it to verify the tach's accuracy and found it to be within 40 RPM at cruise power of 1800. So it turns out my old alternator was giving me an inaccurate reading of 2000 RPM at cruise and now it's accurate. Knowing this made it worth the $30.

I'd send you mine to borrow but it'll cost more than $30 to send it there and back.

In case it is easier....I bought the same tach on Amazon and i think it was a couple of bucks less.

__________________"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

I found my alternator driven electrical tach is 10 percent fast across the whole scale using a $30 photo tach. IE 1800 rpm reading is 1620 actual, which is my normal cruising speed of 6.7 knots. I'm just not ready to fool around trying to adjust a pair of 30 year old tachs.

I check my tachs every season and adjust them if necessary. I had to adjust them both last season when I changed to a Balmar alternator.
It's really not that hard to do. I set the engine to somewhere around 1800 rpm then adjust until I get within about 25-50 rpm. That's close enough.
Then I check them at idle speed and they stay within that 25-50 rpm range.
My experience is that once they are set they hold pretty well.

I check my tachs every season and adjust them if necessary. I had to adjust them both last season when I changed to a Balmar alternator.
It's really not that hard to do. I set the engine to somewhere around 1800 rpm then adjust until I get within about 25-50 rpm. That's close enough.
Then I check them at idle speed and they stay within that 25-50 rpm range.
My experience is that once they are set they hold pretty well.

Same here. Our adjustment is on the back. I checked our $11 tach against a $50 tach and it was right on. If the idle speed is off, the tach's mechanical zero or needle rest could be off and that does take some disassembly to set the needle stop. Not hard but just a heads up.

Here's a tutorial that Dave on fryedaze did that shows the disassembly of a VDO tach.

IF accurate RPM is required ask the engine builder how to hook up a tach sender , and purchase a tach that can read the sender.

After adjustment , this gives most accurate readings .

We have always had issues with the alternator driven tach on our Lehman. Once the batteries are charged the tach pulses as the regulator turns on and off.
Any recommendations for a tach/sender combination for this engine?

Same on my Perkins, Doug. When I increase the electrical load on the batt, the increased alternator output stabilizes the tach. If it's the start bank, I close (switch on) the combiner to share the charges and loads.

I was at Brighouse electric with my winch the other day. They work on starter motors, alternators etc, and they put the double pulley on my alternator last spring. I raised this issue with them and they confirmed what has been mentioned here...that alternator tachs are unreliable and there are too many variables that affect the accuracy. I will probably give them a quick calibration if I can and get a more reliable instrument. That "Tiny Tach shoukd like one option.

Also, it seems to me that engine hours might be affected too. I noticed that sometimes my hour meter indicates more hours at the end of my trip than my recorded hours in my logbook, and the engine isn't sitting around idling more than 15 minutes or so bed or we get underway.

I found a digital laser tach on Amazon.ca, for $44 Cdn, including express shipping. More than what it would cost in the US, but I got it nonetheless. I wasn't able to find one at Canadian Tire or Lordco. Princess auto had one advertised for $60, but none in stock.

Ok. The tachometer is a Motorola model. I dropped a panel down and the back of the tach looks like page 2 of this doc., and I've located the calibration screw, so should be able to figure it out. I'm going to adjust the calibration so it is accurate for 1800 rpm, and then measure rpms at each 200 increment on the dial to WOT.

I got one of those Laser Tachometers from Amazon and it works like a hot damn! Here's the calibration curve and it's not linear (no surprise). It's probably some sort of a quadratic relationship. I shall have to see, but I will calibrate the tach at 1800 and resample at different readings.

Yes, but the % deviation is increasing with RPM. I will calibrate the tach for 0 deviation at 1800 and check the deviations at low idle and at 2400. WOT was 2650 when the tach was accurate before I changed the alternator pulley and threw it off.

No, I already did that last year. I was advised by several people, (a mechanic, a shipwright and the alternator repair outfit I go to) to put a double belt on my alternator and that required installation of a double pulley. That put the calibration of the tachometer "out of wack" and I pretty much guesstimated the cruising RPM's last summer. I figured that that 1600 on the tach was actually 1800, and I was close based on the measurements I took today.

Approximating the engine RPMs from the alternator isn't ideal, I now realize.